Cardiac monitor

ABSTRACT

The monitoring device is self-contained, compact and is preferably strapped about the wrist over the region where a radial pulse is normally detected. Heart rate and abnormalities in the heart beat rate are sensed by the device. Because no external connections are necessary to a power source, for example, the device is particularly useful for ambulatory patients suffering from various types of heart ailments. A pressure sensitive diaphragm sensor is held against the skin over the radial pulse region to sense the pulse wave transmitted from the heart. This pressure sensor couples to a mechanoelectrical transducer which converts the pressure pulse into an electrical analog pulse. A multivibrator, or the like, couples from the transducer and is used to square (digitize) the analog pulse. Logic circuitry receives a digital heart pulse from the multivibrator and is adapted to detect when the heart rate is either below a minimum predetermined rate or above a maximum predetermined rate. When either of these conditions exists an alarm signal is generated. Circuitry may also be included for those patients who may be expected to experience irregularities of the heart beat. This circuitry determines when heart beats have been dropped, for example.

United States Patent [1 Manuel et al.

1 CARDIAC MONITOR [76] Inventors: Barry Manuel, Lockeland Rd.,

Winchester, Mass. 01890; Harvey Pastan, 60 Shaw Rd., Chestnut Hill,Mass. 02167 22 Filed: Mar. 7, 1973 [21] Appl. No.: 338,734

Related US. Application Data [62] Division of Ser. No. 107,959, Jan. 20,1971, Pat. No.

[52] US. Cl. l28/2.05 11, 73/398 C, 317/246, 324/61 P [51] Int. Cl A6lb5/02 [58] Field of Search 128/205 P, 2.05 E, 2.05 H, 128/205 N, 2.05'R,2.05 S; 324/61 P; 317/246; 340/200; 73/398 C 1451 Oct. 1, 1974 PrimaryExaminer-Richard A. Gaudet Assistant Examiner-lee S. Cohen [5 7]ABSTRACT The monitoring device is selfcontained, compact and ispreferably strapped about the wrist over the region where a radial pulseis normally detected. Heart rate and abnormalities in the heart beatrate are sensed by the device. Because no external connectionsarenecessary to a power source, for example, the device is particularlyuseful for ambulatory patients suffering from various types of heartailments.

A pressure sensitive diaphragm sensor is held against the skin over theradial pulse region to sense the pulse wave transmitted from the heart.This pressure sensor couples to a mechano-electrical transducer whichconverts the pressure pulse into an electrical analog pulse. Amultivibrator, or the like, couples from the transducer and is used tosquare (digitize) the analog pulse. Logic circuitry receives a digitalheart pulse from the multivibrator and is adapted to detect when theheart rate is either below a minimum predetermined rate or above amaximum predetermined rate. When either of these conditions exists analarm signal is generated. Circuitry may also be included for thosepatients who may be expected to experience irregularities of the heartbeat. This circuitry determines when heart beats have been dropped, forexample.

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CARDIAC MoNtToR RELATED APPLICATIONS The present invention is adivisional application of application Ser. No. 107,959, filed Jan. 20,1971, now U. S. Pat. No. 3,742,937.

FIELD OF THE INVENTION The present invention relates in general to adevice for monitoring the beat of the human heart. More particularly,the present invention is concerned with a cardiac monitor that sensesthe heart beat and is adapted to cause an alarm condition when the heartrate is either too fast, too slow, or irregular. The device of thisinvention can be constructed compactly and thus is preferably used byambulatory heart patients suffering from various types of heartailments.

BACKGROUND OF THE INVENTION Nowadays there are many patients sufferingfrom heart disease who have conduction problems; that is, problems oftransmitting the electrical impulse from the sinus node of the heart tothe ventricle. To aid these patients a cardiac Pacemaker (RegisteredTrademark) is surgically placed in the body. Some of the Pacemakers areof the demand-type which are designed to provide a rate that is adequatefor the particular activities of the patient. One of the problems inimplanting Pacemakers has been the occasional failure or prematuredischarging of the batteries associated with the Pacemaker, which arealso implanted beneath the skin of the patient. When a battery failuredoes occur 'it is sometimes difficult for the patient to recognize themalfunction unitl a major catastrophe has occurred.

Hospitals are provided with testing devices for monitoring heart rate.However, usually these devices are quite bulky. The real problemassociated with these essentially stationary testing devices is thatthey are intrusive and are not adapted to be worn by the patient outsideof the hospital.

OBJECTIVES OF THE INVENTION One important objective of the presentinvention is to provide an improved cardiac monitor that senses when therate of beat of the heart is either below a predetermined minimum valueor above a predetermined maximum value. These minimum and maximum rateswould be determined by the patients physician who would consider suchfactors as the particular ailment involved and the past history of thepatient.

Another important objective of the present invention is to provide animproved cardiac monitor that senses abnormalities or irregularities inthe rate of the heart beat such as the heart dropping one or more beats,or the occurrence of double heart beats (multiple premature beatsoccurring either singularly or in a run).

Another objective of the present invention is to provide a cardiacmonitor that employs a pulsed light source which is visible to thepatient and which is repetitively activated by each heart beat when thepatient wants an immediate indication of his present heart rate.

A further objective of the present invention is to provide a cardiacmonitor that is self-contained, preferably strapped about the wrist likea wristwatch, and is relatively light in weight and compact.

SUMMARY OF THE INVENTION According to the present invention, the cardiacmonitor attaches to an appropriate point on the patients body such as apressure point where the pressure pulses transmitted from the heart canbe sensed. In a preferred embodiment of the invention the cardiacmonitoring device is strapped about the patients wrist like awristwatch, and is adapted to generate an alarm indication when theheart beat rate is less than a minimum predetermined value, more than amaximum predetermined value, or when the heart beat rate occurs atabnormal intervals. This device is quite useful for an ambulatorypatient with an implanted pacemaker, especially when the patient is notnear a hospital or doctor. The patient would be alerted as soon as hispulse rate went above or below the predetermined levels, which levelswould be preset by his physician. Should such an alarm condition occur,the patient would immediately contact the physician so that appropriatecorrection can be made before any serious injury occurs.

In accordance with another aspect of the present invention, the devicemonitors abnormal activities of the heart. It is well known that peopleunder severe stress manifest this stress in a number of ways concerningheart functioning. For example, the heart may occasionally either drop abeat or produce a double beat. The cardiac monitor is thus preset forpatients expected to exhibit this type of arythmia so that an alarmindication occurs when a certain number of irregulaties are detectedwithin a given time period.

A cardiac monitor constructed according to thisinvention basicallyincludes a pulse pressure sensor, a mechano-electric transducer andalarm circuitry including logic circuitry. The pressure sensor may be astrain gauge sensor or a variable capacitance sensor. It is preferredthat the sensor be more sensitive to pressure changes of the typenormally monitored from the heart, and not be sensitive to slow pressurevariations. The mechano-electric transducer couples to the pressuresensor and converts the sensed pressure pulse into an electrical analogpulse. In the disclosed embodiment the mechano-electric transducerincludes a known circuit that operates using a differential pulse-widthmodulation scheme, and a low-pass filter that smooths the pulsed outputto an analog output signal. A monostable multivibrator or Schmitttrigger circuit may then be coupled from the mechano-electric transducerto provide a digitized (square) output of controlled width. Logiccircuitry couples from the digital output of the monostablemultivibrator and is adapted to generate an alarm signal when the rateof the heart beat is below a minimum predetermined rate or above amaximum predetermined rate. This alarm signal (digital level) may thenbe fed to a conventional alarm.

in accordance with another aspect of this invention, the cardiacmonitoring device may include alarm circuitry for detecting abnormalfunctioning of the heart. This circuitry may include a complementingflip-flop that is set and reset on alternate heart pulses. Additionalcircuitry connects to the output of the flip-flop and essentiallymeasures the time that the flip-flop is in each state. When the heart isoperating normally the flip-flop should be in each state forapproximately equal amounts of time. When one or more beats are dropped,for example, the output circuitry detects this condition, as theflip-flop will remain in one of the two states longer, and an alarmindication is generated.

BRIEF DESCRIPTION OF THE DRAWINGS Numerous other aspects of thisinvention along with additional objectives, features and advantages ofthe invention should now become apparent upon a reading of the followingexposition in conjunction with the accompanying drawings in which:

FIG. 1 shows a cardiac monitor of this invention attached to thepatients wrist to monitor radial pulses;

FIG. 2 is a block diagram of a cardiac monitoring device constructed inaccordance with the principles of the present invention; I

FIG. 3 is an enlarged cross sectional view of one embodiment of apressure sensor;

FIG. 4 is an enlarged cross sectional view of a preferred embodiment ofa pressure sensor;

FIG. 5A is a circuit diagram of one embodiment of the mechano-electricaltransducer of FIG. 2, including the monostable multivibrator, also shownin FIG. 2;

FIG. 5B shows various waveforms associated with different points in thecircuit of FIG. 5A;

FIG. 6 is a logic block diagram illustrating one embodiment for the ratelogic of FIG. 2;

FIGS. 7A, 7B, and 7C show waveforms associated with the diagram of FIG.6 for a no alarm condition, for a less than minimum predetermined ratealarm condition, and for a greater than maximum predetermined rate alarmcondition;

FIG. 8A shows a diagrammatic representation of a complementing flip-flopused in the diagram of FIG. 6;

FIG. 8B shows various logic gates that may be used in the cardiacmonitor;

FIG. 9 is a circuit diagram of the irregularity circuitry depicted inFIG. 2; and

FIG. 10 is a logic block diagram for another embodiment of theirregularity circuitry of FIG. 2.

EXPOSITION Referring now to the drawings, and in particular to FIG. 1,there is shown a cardiac monitor 10 con structed in accordance with thisinvention and strapped by suitable means about the wrist l I of thepatient. The cardiac monitor 10 may include most of the circuitry shownin the block diagram of FIG. 2. The strap 12 extends about the entirewrist. In FIG. 1 the radial pulse area of the wrist is not shown.However, the strap 12 is used to hold a pressure sensor against theradial pulse region in order to sense pressure pulses transmitted fromthe heart.

A logic block diagram of one embodiment of the cardiac monitor is shownin FIG. 2. The sensed pressure pulse is represented by the block I4.This pressure pulse is also depicted in FIG. 5B. The pressure pulse iscoupled through the patients skin to the pressure sensor I6, twoseparate embodiments of which are shown in FIGS. 3 and 4, and discussedin more detail hereafter. One or more output connections from pressuresensor 16 couple to mechano-electric transducer which converts thesensed pressure pulse into an electrical analog pulse that isrepresentative of the pressure pulse. In the disclosed embodiment ofFIG. 5A the mechano-electric transducer includes a known circuit thatoperates using a differential pulse-width modulation scheme, and alow-pass filter that smooths the pulse output to the analog outputsignal. The output of transducer 30 couples to monostable multivibrator46 whose output provides a digitized (squared) voltage output ofcontrolled width. The output of multivibrator 46 may be referred to asthe digital heart output. It is this output that is processed by thealarm circuitry of FIG. 2.

The alarm circuitry includes rate logic 50, irregularity circuitry 70, asystem clock 80, and an alarm 90. Rate logic 50 receives the digitalheart output signal from multivibrator 46, and is adapted to generate analarm signal on output line 50A when the rate of the heart beat is belowa minimum predetermined rate or above a maximum predetermined rate. Inthe illustrated embodiment the system clock 80 provides a negative-goingground pulse every I5 seconds which resets a counter in rate logic 50.(See FIG. 6). Thus, system clock 80 provides timing that enables ratelogic 50 to observe heart beats and sense alarm conditions in 15 secondintervals. FIG. 2 also shows pulsed light source 92 (see also FIG. 1)which may be activated by a momentary push button 93 and allows thepatient to count the light pulses representative of each heart beatduring a 15 second interval, to thereby determine visually what hisheart rate is at any given time.

The irregularity circuitry of FIG. 2 receives the digital heart pulsefrom multivibrator 46 and is adapted to determine whether an abnormalfunctioning of the heart has occurred. For example, the heart mayoccasionally either drop a beat or produce a double beat. Should acertain number of predetermined irregularities occur within the 15second interval, for example, irregularity circuitry 70 would generatean alarm signal on output line 70a which is coupled to alarm 90. In FIG.2 a dotted line is shown from the system clock to irregularity circuitry70. For some applications this connection is not necessary. The detailsof circuitry 70 are discussed in more detail with reference to FIG. 9. Alogical arrangement for circuitry 70 is depicted in FIG. 10.

In FIG. 1 the cardiac monitor is shown strapped to the patients wristand the pressure sensor is held against the radial pulse region. Inanother arrangement the pressure sensor could surround the patientsfinger in order to detect the digital pulse. Alternatively, a specialeyeglass frame could be designed to support the pressure sensor so thatit could detect the temporal pulse. The device could be worn around theankle so as to monitor the posterior tibial pulse or around the foot todetect the dorsalis pedis pulse.

FIG. 3 shows one embodiment for a pressure sensor 16 employingsemiconductor strain gauges 17. The strain gauges 17 are diffused withinsilicon diaphragm 18 which may be annular in shape. Diaphragm 18includes a thin central portion 19 having an orifice 20 centrallydisposed therethrough. Silicone rubber capsule 21 surrounds diaphragm l8and defines chambers 22 and 23 above and below the central portion 19 ofdiaphragm 18. A silicon oil 24 fills both chambers 22 and 23.

The four strain gauges 17 are connected in a full Wheatstone Bridgeconfiguration with a resistance of between 1,000 and 10,000 ohms. Thetwo excitation and two signal connections comprise the four leads 15.With the embodiment of FIG. 3 a relatively powerful battery source isneeded to excite the strain gauges 17. The output from the bridgecircuit is amplified using a low voltage microwatt powered integratedcircuit amplifier such as the Fairchild FuA735C. The amplifier outputwould be biased to the logic zero state when the differential pressureis zero, and when a minimum pressure pulse is received the output risesto the logic I state producing a pulse from a multivibrator connected tothe amplifier output. This multivibrator output could then be used as adigital heart pulse.

However, because excess power may be necessary to operate the embodimentof FIG. 3, the pressure sensor of FIG. 4 is a preferred embodiment inaccordance with the principles of this invention. The pressure sensor116 of FIG. 4 is a variable capacitance sensor that includes fixed endplates 25 and 26 which are constructed of a conductive material and maybe annular in shape. A conductive pressure sensitive diaphragm 27 isdisposed by means of insulating spacers 25a, 26a intermediate fixed endplates 25 and 26. The plates 25 and 26 each have a plurality ofcomparatively large holes 25b and 26b, respectively, extendingtherethrough. Diaphragm 27 contains one centrally disposed aperture 27a.A silicon rubber capsule 21a encloses plates 25 and 26, and diaphragm27, and defines chambers 22a and 23a above and below fixed plates 25 and26, respectively. A silicone oil 24a fills chambers 22a and 23a,- and isalso on both sides of diaphragm 27.

Three leads a connect from plates 25 and 26 and from diaphragm 27 to thecircuit shown in FIG. 5A, which schematically shows the capacitor sensorat 311.

FIG. 4 is an enlarged view of the capacitor pressure sensor. In anactual embodiment the diameter of the sensor is between 1/4 inch and 1/2inch and may have a thickness of 0.1 to 0.2 inch. This sensor is smallenough to be easily mounted immediately over the artery of interest witha comfortable clamping pressure suitable for continuous wearing by thepatient.

In FIG. 4 when a pressure pulse is applied in the direction of arrow 29the diaphragm 27 deflects twoard plate 25., Previous to this deflectionthe capacitance between diaphragm 27 and plate 25 was approximatelyequal to the capacitance between the diaphragm and plate 26. However,when the diaphragm deflects the capacitance between diaphragm 27 andplate 25 increases and the capacitance between the diaphragm and plate26 decreases. It is this change in capacitance that is sensed by thecircuitry of FIG. 5A.

For the application of this invention it is desirous that a leakydifferential pressure sensor which is primarily sensitive only topressure changes be used. Such a device is provided in the embodiment ofFIG. 4 wherein the sensor 116 is eancapsulated in a soft silicone rubberwith the diaphragm 27 defining chambers on either side thereof connectedonly by orifice 27a. Filling both chambers with a low viscosity siliconeoil allows coupling of the pressure fluctuations through the chamberwalls to the diaphragm. The arrangement of FIG. 4 is' also advantageousin that is is not sensitive to slow steady-state variations, such asmight be caused by shifting the clamping pressure of the device.

It would be possible to make capacitance sensor 16 with one fixed plateonly in which case the other plate would be replaced by a fixedcapacitor. This scheme results in less output for a given diaphragmdeflection but may be useful in certain applications.

The pressure pulse that is capacitively sensed by the pressure sensor ofFIG. 4 is converted into an analog voltage proportional to differentialpressure by using a differentil pulse-width modulation scheme such asthat described in U. S. Pat. No. 3,518,536. The circuitry in FIG. 5A issimilar to that disclosed in the above mentioned patent. The waveformsof FIG.'5B are associated with the circuitry of FIG. 5A.

The circuitry of FIG. 5A generallyincludes a capacitive pressure sensorindicated at 311, a flip-flop 32, comparators 34 and 36, and one shotmultivibrator 46. Flipflop 32 is of conventional design and has its Qoutput coupled by way of resistor R1 and diode D1 to plate 25 of thepressuretransducer. The Q of flip-flop 32 couples by way of resistor R2and diode D2 to the other plate 26 of the pressure transducer. Thediaphragm 27 is coupled by way of capacitor C1 to the +Vcc power supply.The cathodes of diodes Dll and D2 couple respectively toone input ofcomparators 34 and 36. The other input to each of the comparators 34 and36 is an E reference signal which is typically a dc voltage ofapproximately l/2 Vcc. The outputs of comparators 34 and 36 couplerespectively to the set (S) and reset (R) inputs of flip-flop 32.

In operation, flip-flop 32 is alternately set and reset through thecharging circuit includingcomparators 34 and 36, and the capacitance ofthe pressure transducer. Assuming that the spacing of the diaphragm 27is halfway between plates 25 and 26 the flip-flop 32 should be in eitherstate equal amounts of time. Thu when flip-flop 32 is set its 0 outputis high and its Q output is low. A discharge path is provided by way ofcapacitor Cll, plate 26 and resistor R2 to the 6 output of flip-flop 32.As the voltage on fixed plate 26 decreases comparator 36 is enabled whenthis voltage reaches the E reference voltage and flip-flop 32 becomesreset. When this action occurs the Goutput becomes positive and the Qoutput goes to ground. The discharge path is then through resistor R1and capacitor Cll until comparator 34 is enabled and the flip-flop 32then reverts to its set state.

The top waveform of FIG. 5B shows the. pressure pulse that deflectsdiaphragm 27 to alter the capacitance of the pressure transducer. It isalso noted in FIG. 5B that the waveform at point A (0 output offlip-flop 32), indicates that flip-flop 32 is in its set state for ashorter time interval than it is in its reset state. This mode ofoperation can easily be provided by disposing the diaphragm 27 closer tothe plate 25 so that the capacitance therebetween is larger than thecapacitance between diaphragm 27 and plate 26. Thus, whenflipflop 32 isreset (Q output at ground) the discharge path is through resistor Rlland the charging time constant is relatively long so that the 0 outputstays at ground for a longer period than it stays at its positivevoltage.

When the pressure pulse is sensed diaphragm 27 deflects downwardlytending to equalize the capacity between diaphragm 27 and the two plates25 and 26. When this occurs the waveform shown in FIG. 5B changes sothat the voltage level at the 0 output stays at the positive voltagelevel longer than at the ground level. The waveform at A is a variablepulse width waveform modulated by the value of the sensed pressure.

The waveform at A is coupled to a filter including resistor R3 andcapacitor C2 which smooth the waveform as indicated at point B. This isa low-pass filter. The waveform at B is coupled to one shotmultivibrator 46,

and when the voltage at B reaches the required threshold voltage ofmultivibrator 46, the multivibrator switches to its positive state for aperiod of about .05 seconds as indicated in the bottom waveform of FIG.5B. Multivibrator 46 may be of conventional design, and its Q output ishereinafter referred to as the digital heart output or heart beatoutput.

One embodiment for the rate logic 50 depicted in FIG. 2 is shown inblock diagram fashion in FIG. 6. Rate logic 50 includes a six stagebinary counter 52, an alarm flip-flop 54, and a light flip-flop 56. Thewaveforms shown in FIGS. 7A, 7B and 7C are associated with the circuitryof FIG. 6.

In the disclosed embodiment a 15 second observation time has been chosenduring which digital heart pulses are received by rate logic 50. It mayfurther be assumed that the minimum predetermined rate is 40 beats perminute and that the maximum predetermined rate is I44 beats per minute.When these maximum and minimum values are translated to a 15 secondinterval their values are 10 and 36 beats, respectively, per 15 secondinterval.

Binary counter 52 is shown somewhat simplified, and includes stages58-63. Each of these stages may be of the complementing flip-flop typeas indicated in FIG. 8A. By connecting the Q output from each of theseflipflops to the clock input of the next flip-flop, a binary counter maybe constructed. The resetting of counter 52 to all zeros is accomplishedevery 15 seconds by a ground going pulse of approximately 10milliseconds duration from system clock 80. This ground pulse is coupledto all of the DC reset inputs of the stages 58-63 in order to resetcounter 52 every 15 seconds and start a new count.

The digital output which has a duration of approximately .05 seconds iscoupled to the clock input of stage 58, and each heart beat pulse thatis received increments the counter 52 one count.

The outputs from counter 52 couple to two NAND gates 64 and 65. The twoinput connectors to gate 64 couple from the output of stage 59 and theoutput of stage 61. Thus, gate 64 decodes a code of IO (2+8).

The output of gate 64 couples to the DC set input of alarm flip-flop 54.An inverter 66 couples from the system clock input to the clock input ofalarm flip-flop 54. A positive pulse of approximately 10 millisecondsduration is therefore impressed upon the clock input of flipflop 54every fifteen seconds to reset the flip-flop if it has been previouslyset.

Referring now to FIG. 7A there are shown various waveforms for the heartbeat, the 15 second system clock, the outputs of stage 58 and alarmflip-flop 54, and the condition of the alarm output 67. When ten beatsare counted in less than l5 seconds the gate 64 is enabled and itsoutput goes to ground. This action sets alarm flip-flop 54 and its 6output goes to ground. The 6 output of flip-flop 54 couples to one inputof NAND gate 68. The other input to NAND gate 68 is the inverted systemclock which is positive going for a l() millisecond internal every 15seconds. Thus, when the alarm flip-flop 54 is set the gate 68 cannot beenabled and its output remains at a logical one level. This logical onelevel is coupled to the alarm gating 69 and keeps the output 67 at aground level, which level is a no alarm level. i

The output of gate 66, besides enabling gate 68, resets alarm flip-flop54 every l5 seconds on its trailing (negative-going) edge.

Referring now to FIG. 7B there are shown waveforms similar to thoseshown in FIG. 7A for a condition where less than 10 beats are receivedduring the IS second interval. This should cause an alarm condition asindicated in the lower waveform of FIG. 7B. Thus. if less than 10 beatsare counted during the 15 second interval, gate 64 is not engbled andalarm flip-flop 54 remains reset with its 0 output at a logical onelevel. When the inverted system clock input to gate 68 goes positive thegate 68 is enabled and its output reverts to a logical zero level. Thisaction enables gate 69 and the alarm output 67 goes positive. Actually,any zero logical level to gate 69 will cause an alarm condition at alarmterminal 67.

FIG. 7C shows various waveforms associated with the circuitry of FIG. 6for a condition where more than 36 beats are detected during the 15second observation interval. When counter 52 reaches a count of 36 gate65 is enabled and its output which couples directly to alarm gate 69goes to a zero voltage level thereby causing an alarm signal on terminal67.

The alarm output at terminal 67 may be connected to a power driver (notshown) which energizes the magnetic detent holding the mechanical springactuated alarm, and thereby produce an audible alarm similar to thealarm used in many conventional wrist alarm watches, when the signal onterminal 67 goes positive and the magnetic detent is energized and nolonger holding the spring. With this type of an arrangement the alarmcontinues to operate even though all of the inputs to gate 69 from gates65 and 68 revert to their logical one levels. In order to reset thealarm 67 an alarm reset push button 69a is coupled from ground to gate69. This alarm reset switch 690 need only be a momentary contact switch.Thus, the alarm is held on by means of the output alarm latch anddepressing switch 69a will cause the mechanical arm to turn off byswitching on the latch.

Another feature of the present invention illustrated in FIG. 6 isprovided by the circuitry including flip-flop 56 which produces avisible light pulse for each heart beat measured during a 15 secondinterval. Flip-flop 56 receives an inverted system clock pulse at itsclock input terminal and also has an input from a +5 volt supply whenswitch 93 is depressed. The Q output of flipflop 56 is normally at alogical zero level due to the +Vcc on the K input to flip-flop 56.However. when switch 93 is closed and the inverted system clock pulse isreceived by flip-flop 56, the Q output reverts to a logical one leveland remains in that state during the next 15 seconds until the flip-flopreceives the next system clock pulse. The Q output of flip-flop 56 andthe digital heart beat pulse are coupled to AND gate 71. When flip-flop56 is set and a digital heart beat pulse is received, gate 71 is enabledand current flows to light emitting diode (LED) 72 thereby causing it toflash once for each detected heart beat pulse.

Once the light emitting diode 72 starts flashing the momentary switch 93(see FIG. 1) can be released and the flashing will continue for 15seconds until flip-flop 56 is reset by the inverted system clock pulsethereby reverting the Q output of flip-flop 56 to its logical zero stateand inhibiting gate 71. The patient may count the flashes from diode 72and multiply by four to obtain his present heart rate in beats perminute. This feature provides a convenient way for the patient to checkhis pulse and also check the operation of the instrument. The diode 72is quite rugged, extremely small, visible in strong light and requiresonly 10 milliamps from a low voltage supply and has an extremely longlife span.

With the arrangement shown in FIG. 6 it is quite easy to change the lowrate and high rate levels. In the example previously given the low ratewas 10 and the high rate 36. However, it is quite easy to change this todifferent levels by connecting different outputs from counter 52 to thegates 64 and 65.

One embodiment for the irregularity circuitry 70 of FIG. 2 is shown in acircuit diagram of FIG. 9. The circuitry of FIG. 9 generally includes acomplementing flip-flop 74, charging circuit 75, and output differentialamplifier 76. Each digital heart pulse from multivibrator 46 is coupledto the clock input of flip-flop 74, and this flip-flop sets and resetson alternate pulses. If the input pulse is received at a constantfrequency the flipfiop 74 should be in each state an equal amount oftime.

Charging circuit 75 couples to the output of flip-flop 74 and includesseries resistors R5 and R6 coupled from the Q output to one input ofamplifier 76, and series resistors R7 and R8 coupled from the Q outputto a second input of differential amplifier 76. A capacitor C5 couplesacross resistors R5 and R7. A capacitor C6 and resistor R9 are tied inparallel from the first input to differential amplifier 76 to the outputof amplifier 76.

When a rhythmic constant frequency pulse is received the net chargeacross capacitor C5 should be approximately zero volts. Capacitor C5along with resistors R5 and R7 actually form a filter having a timeconstant of 2 R5 C5 where R5 and R7 have equal values. In one designthis time constant was approximately 30 seconds.

When an alarm condition exists, such as when the heart drops a beat,flip-flop 74 will remain in one state for a longer period than itremains in the other state. This means that capacitor C5 will be chargedeither positively or negatively depending upon the longer state offlip-flop 74. In either case a sufficient positive or negative chargecoupled to the input of differential amplifier 76 will cause theamplifier to direct a current through alarm detent 77 which isresponsive to currents in either direction. The gain of the comparatoramplifier 76 is equal to R9/R6 and this value is usually low dependingupon the drive requirements of the magnetic detent on the mechanicalalarm. A typical closed loop gain is between 0.1 and 10 times. CapacitorC6 and resistor R9 are used to provide additional smoothing of theoutput and normally have a time constant on the order of 10 to l5seconds. The value of resistor R9 may be increased to thereby increasethe sensitivity of the system to arythemia. whereas decreasing the valueof R9 will decrease the system sensitivity. The proper value forresistor R9 may be chosen after knowing a particular patients condition.

Another useful feature to the circuitry of FIG. 9 is that it isessentially fail safe should the transducer become dislodged orinoperative. Under these conditions the circuit of FIG. 9 will produce asteady state output from either the Q or the 6 outputs which will veryrapidly charge capacitor C5 either positively or negatively.

This action in turn causes an amplifier output that will energize thealarm detent 77. I

Referring now to FIG. 10 ther is shown a logical implementation foranother embodiment of the irregularity circuit 78 of FIG. 2.Thecircuitry of FIG. I0 generally includes a complementing flip-flop 82,an up/down counter 84 and comparators 86 and 88. All of the circuitryshown in FIG. 118 may be of conventional design. The digital heart pulseis coupled to the clock input of flip-flop 82, and this flip-flop willalternate between its set and reset state. The Q output of flip-flop 82couples to AND gate 89, whereas the 0 output couples to AND gate 91. Inthis embodiment a 100 cycle per secnd clock is used which indirectlycounts the counter 84. This clock output is coupled to the other inputsof gates 89 and 91.

Thus, when flip-flop 82 is set, gate 89 is enabled and pulses from clock92 are passed by way of gate 89 to the up count line 92a of counter 84.Similarly, when flip-flop 82 is reset gate 911 is enabled and 100 cycleper second pulses from clock 92 are passed to down count line 92b ofcounter 84. The counter 84 may also be provided with an input from thesystem clock which resets the counter to a zero count every 15 seconds.

Counter 84 is of the type that will count up when a pulse is received online 92a and will count down when a pulse is received on line 92b. Itmay be assumed that the intiial count in the counter is zero. Thepositive output 93 of counter 84 will show a positive count when morepulses have been received on line 92a. Similarly output lines 94 willhave a negative count when more pulses have been received on line 92b.Actually, the outputs 93 and 94 would include a plurality of outputlines that would be coupled to comparators 86 and 88, respectively. Apositive reference count block 95 and a negative reference count block96 would have predetermined counts set therein and would have theiroutputs coupled to comparators 86 and 88, respectively. The output ofthe comparators would couple to OR gate 97 and from there to an alarmcircuit which could be of conventional design. Thus, when eithercomparator 86 or 88 detects a sufficient positive or negative countequal to the count in blocks 95 or 96, respectively, an alarm signal iscoupled via either input of OR gate 97 to the alarm.

In the foregoing exposition reference has been made to abnormalities inthe functioning of the heart, such as the heart missing a beat or havinga double beat. These abnormalities are common occurrences in somepatents. When the heart is beating at a rate of one beat per second, forexample, a beat is missed when the time interval between beats becomestwo seconds or more. Similarly, a double beat may happen when two beatsoccur with a time interval therebetween on the order of 0.1 second, forexample. i

What is claimed is:

I. A pressure sensor for detecting a pressure pulse and comprising;

a resilient capsule having means forming a hollow inner chamber,

at least one conductive plate extending across the chamber and having anopen area through the plate and being relatively immovable relative tothe capsule,

a flexible electrically conductive diaphragm disposed in the chamber andhaving an orifice therethrough.

said sensor being responsive to a pressure pulse coupled to theresilient capsule to cause the diaphragm to deflect thereby varying thecapacitance between the plate and diaphragm. 2. The pressure sensor ofclaim 1 comprising a second conductive plate disposed in the chamber andmeans for insulatedly spacing said second plate from said diaphragm andon a side of said diaphragm opposite the first plate, said conductormeans also coupling from said second plate.

3. The pressure sensor of claim 2 wherein said second plate also has anopen area therethrough, both plates having fluid surrounding them.

4. The pressure sensor of claim 3 wherein said means for insulatedlyspacing includes aplurality of spacers.

5. The pressure sensor of claim 4 wherein the open area of both platesis defined by a plurality of holes.

6. The pressure sensor of claim 5 wherein the orifice is centrallydisposed in the diaphragm.

7. The pressure sensor of claim 6 wherein said conductor means includesthree wires, one connecting from the first plate, one from the secondplate and one from the diaphragm.

8. The pressure sensor of claim 1 including a flxed capacitor, saidconductor means also coupling from said

1. A pressure sensor for detecting a pressure pulse and comprising; aresilient capsule having means forming a hollow inner chamber, at leastone conductive plate extending across the chamber and having an openarea through the plate and being relatively immovable relative to thecapsule, a flexible electrically conductive diaphragm disposed in thechamber and having an orifice therethrough. means for insulatedlyspacing said plate and diaphragm, a fluid contained in the chamber andsurrounding the plate and diaphragm, said orifice in said diaphragmbeing smaller than the open area through the plate and being smaller incomparison to the volume of fluid so that the diaphragm is relativelyinsensitive to slow pressure variations, and conductor means couplingfrom the plate and diaphragm for carrying electrical signalfluctuations, said sensor being responsive to a pressure pulse coupledto the resilient capsule to cause the diaphragm to deflect therebyvarying the capacitance between the plate and diaphragm.
 2. The pressuresensor of claim 1 comprising a second conductive plate disposed in thechamber and means for insulatedly spacing said second plate from saiddiaphragm and on a side of said diaphragm opposite the first plate, saidconductor means also coupling from said second plate.
 3. The pressuresensor of claim 2 wherein said second plate also has an open areatherethrough, both plates having fluid surrounding them.
 4. The pressuresensor of claim 3 wherein said means for insulatedly spacing includes aplurality of spacers.
 5. The pressure sensor of claim 4 wherein the openarea of both plates is defined by a plurality of holes.
 6. The pressuresensor of claim 5 wherein the orifice is centrally disposed in thediaphragm.
 7. The pressure sensor of claim 6 wherein said conductormeans includes three wires, one connecting from the first plate, onefrom the second plate and one from the diaphragm.
 8. The pressure sensorof claim 1 including a fixed capacitor, said conductor means alsocoupling from said capacitor.